There are mainly two kinds of modern light water reactors: boiling water reactors (BWR) and pressurized water reactors (PWR). In these kinds of reactors different conditions exist. Therefore there are different requirements on the parts that are included in the different kinds of reactors. In a PWR, the fuel rods are cooled mainly by water that is in a liquid phase under a high pressure. In a BWR, the pressure is lower and the water that cools the fuel rods is evaporated such that the fuel rods are surrounded both by water in liquid phase and in a steam phase. Since the water normally flows from below upwards through the fuel assembly, the amount of steam is higher in the upper part of the fuel assembly. Because of the different working principles in a PWR and a BWR, the fuel assemblies have different designs and they differ from each other concerning many details. It is therefore clear to a person skilled in the art whether a certain fuel assembly is for a PWR or for a BWR.
In a fuel assembly for a nuclear boiling water reactor, there are a number of fuel rods, which comprise a nuclear fuel material. As mentioned above, when the fuel assembly is in operation in a nuclear reactor, a cooling medium, usually water, flows up through the fuel assembly. This water fulfils several functions. It functions as a cooling medium for cooling the fuel rods such that they will not be overheated. The water also serves as a neutron moderator, i.e. the water slows down the neutrons to a lower speed. Thereby, the probability that the neutrons induce a fission reaction is increased.
Uranium (“U”) is the predominantly used nuclear fuel in currently operating nuclear reactors. The core of such a nuclear reactor has a large number of fuel assemblies with fuel rods that contain uranium fuel. A certain fraction of the fuel assemblies is removed at regular intervals and replaced with new fuel assemblies in order to compensate for the loss of reactivity occurring with irradiation during the operation of the nuclear reactor.
A uranium-based fuel is normally based on 238U which is enriched in 235U. In a fuel assembly for a BWR the degree of enrichment is usually different for different fuel rods, depending on the fuel rods' position in the fuel assembly. Because of the design of a BWR, the ratio of the volume of the neutron moderating medium (water) to the volume of the fuel is different for different positions in the fuel assembly. The enrichment is therefore varied so that more well-moderated fuel rods have a lower enrichment and less well-moderated fuel rods have a higher enrichment.
The nuclear fuel material in some of the fuel rods in a fuel assembly normally includes, in addition to uranium, a burnable absorber, i.e. an isotope having a high neutron absorption cross section. Upon absorption of a neutron, the isotope is converted into an isotope with a low neutron absorption cross section. The purpose of such burnable absorbers is to lower the reactivity of the fuel assemblies while they are new, whereas the reactivity later in life, after the burnable absorber nuclei have absorbed a neutron, is no longer substantially reduced by any burnable absorber. The advantage of using a burnable absorber is that the power distribution between the fuel assemblies in the nuclear reactor core is more even than it would have been in the absence of burnable absorbers. The even power distribution results in higher shutdown margins, since clusters of high reactivity fuel assemblies can be avoided. In addition, a lower power peaking in the core allows for a higher average power level without risk of locally exceeding a certain power limit. The drawback of the use of burnable absorbers is that the power of the fuel rods that contain burnable absorbers is strongly reduced. This results in larger internal power peaking in the fuel assembly, and a slightly lowered reactivity also during the latter part of the life of the fuel assembly, due to remaining absorbing isotopes related to the original burnable absorber isotopes, and due to the reduced mass of fissile U in these fuel rods.
The use of Thorium (“Th”) as a nuclear fuel, mixed with uranium or other fissile materials, has been proposed in numerous patents and academic publications.
WO 85/01826 A1 and WO 97/08711 A2 describe nuclear reactors of the seed-blanket type having an active core comprising seed regions of fissile material and blanket regions of fertile material capable of being converted into fissile material by neutron capture. The blanket regions comprise Th.
U.S. Pat. No. 3,211,621 describes a heterogeneous breeder or converter type neutronic reactor. The reactor core has seed fuel assemblies and blanket fuel assemblies. The blanket fuel assemblies can comprise Th.
GB 903142 describes a PWR. It is described that overheating of fuel elements in regions of enhanced flux caused by a local excess of water moderator is avoided by reducing the concentration of fissile material or by providing a neutron absorber in those regions. In order to avoid overheating in a certain zone (zone I) of the fuel rods, another zone (zone II) contains fuel which is less enriched than a third zone (zone III) and may also contain a large proportion of 232Th, e.g. as ThO2 (“Thorium dioxide”) mixed with UO2 (“Uranium dioxide”).
The use of Th to replace some of the burnable absorbers in uranium fuel assemblies has been proposed in a PhD thesis by Cheuk Wah Lau, ISBN 978-91-7385-990-5. In this work, a fuel assembly for a PWR is proposed in which all of the uranium fuel rods and some of the burnable absorber containing fuel rods are replaced by rods containing enriched uranium and a minor fraction of Th (less than or equal to 50%). The enrichment of the uranium and the fraction of Th is equal for all thorium-containing rods.